In a large cast-iron or other heavy skillet, saute mushrooms in 3 tablespoons butter in batches; set aside. In the same pan, saute onion and celery in 1 tablespoon butter. Add the flour, salt, pepper and remaining butter; cook and stir until smooth., Gradually add water. Bring to a boil; cook and stir until thickened, about 2 minutes. Stir in mushrooms and heat through. Remove from heat; stir in sour cream.